Radio seismic system



Jan. 26, 1954 G. PETERSON RADIO SEISMIC` SYSTEM Filed Aug. 24. 1948 2 Speets-Sheet l '.BY ATTORNEYS r2 fo.

Jan. ze, 1954 Filed Aug. 24. 1948 OUTPUT OF RADIO FREQUENCY AMPLIFIER OUTPUT OF LOCAL OsCILLATOR #I OUTPUT OF LOCAL OSCILLATOR #2 OUTPUT OF LOCAL OsCILLATOR #3 OUTPUT OF LOCAL OsCILLATOR #4 OUTPUT OF LOCAL 52 OSCILLATOR #I 54 OUTPUT OF LOCAL I OsCILLATOR #2 OUTPUT OF RADIO 56 FREQUENCY AMPLIFIER 57 G. PETERSON RADIO SEISMIC SYSTEM 2 Sheets-Sheet 2 INTERMEDIATE FREQUENCY AMPLIFIER COUPLING NETWORK INTERMEDIATE FREQUENCY AMPLIFIER COUPLING NETWORK INVENTOR. GLEN PETERSON gwn/M A TTORNE YS Patented Jan. 26, 1954 UNITED STATES RADIO SEISMIC SYSTEM Glen Peterson, Tulsa, Okla., assignor to Phillips 8 Claims.

vThis invention relates to a radio seismic sys.- tem. In a more specific Vaspect it relates to a seismic system in which the outputs of predetermined groups of seisinometers are combined and simultaneously recorded.

`Ifn Lseismic `prospecting systems, a number of fseismometers 4are arranged in a predetermined pattern and seismic-Waves Yproduced Aby detona- -tion of :an explosive ,charge Ypass from .the shot J-poin't :to the seismometer units which produce electr-ical `voltages representative of .the fseismic `atraves. Theseelectrical voltages, after suitable amplication, .are used 4to -actuate a .recorder and fthe :resulting tracesonthe recording medium `are analyzed to determine :the nature and configura.- -tionoffsubterraneanstrata. Heretofore, the elec- Ytrical .voltages have been .carried from the .seisfmometers 5to `.the recording unit by a series .of rcables .and :these cables substantially limited the flexibilitycf 'the prospecting apparatus, particularly AWhere rough '.or :heavily wooded terrain was encountered. In my lcopending applications, Serial fNos. 41;337, 530257,. and `2'7'5590, Patent Nos. 25479;77'2, 2,494,880 :and 52,557,310, respectively have discloseda complete radioseismic system-.in Lwhich the. cables interconnecting fthe .seismom- :eters-and vrecording -unit are replaced by radio flinkages. This vgreatly increases .the 'eXibility lofzthes seismic systemand permits its use atloca- `tionsiiiifltiere surveys by the .cable connected ap- :zparatus areextremelyfdicult or impossible.

In any of the described r,seismic y. systems, uit :is fotentimesfdesirable that;the outputs oftwoor more :seisdnometers be.v combined and utilized to make a single trace onthe recordingmedium. For eXa-mple, the outputs of a rotational type :seismonieter-f-and atranslational. type seismometer maybemombinedgtoeliminate thefeiiects of Ray- ;leigh or f -surface Waves from the seismograph -:record Aszanother example, severaladjacent r-.Iseismometers :maybe interconnected and their outputs :combined to eliminate; local effects of surface ,-rstrata upon "the lseismorneter outputs. enga-in, two orffmore-seismometers may-becom- :..h-ned to :produce :desired directional effects -so rzthatit becomes possibletol determine the direc- ..tioncfvalrivaloffseismic waves. f With cable connected-systemxgthe portability requirements, the interaction of Zone circuit upon another1 the .c I have discorered a `simple and f convenient :1.55

method .and apparatus for combining theoutputs of .diner-ent seismometers as desired by application of the principles of the radio seismic system. -The present invention 4contemplates placing va number of units, each consisting of a seisniorneter and a radio transmitter, in a predetermined pattern at the region to be surveyed. A multi;- channel receiver of'novel design is utilized to pick up the signals of vthe respective radio transmitters, this receiver providing anoutput voltage proportional to the output of any single seismometer or' of any Clerd QQlDlaiQllS .Q f Sdi@ V. seisniometei's. The output of the receiver is then fed to a device ywhichrecords the voltages repre!- sented by the predetermined combinations of seisinometer outputs.

It `is an Yobiectof the :inventm tapfer/ide a methodf of O.and Qapparatus'-rfortlie migin' t "seflectionof seisni'om'eter tput voltages in one/'or more preselectedcombinations.

It is a kfurther objectl of the inventiontoprovide ai'systemin Avvliich theoutput of ^any seis- `rno'meter can be used several times simultaneously in each seismic pim'blern. Itis ,a still further'fobject of the inventionto provide a system'` for simultaneously recording 'a `Inultiplicity ofdifierentcombinations of agroup Ofseismometersignals! l Itis a stillA further object of the invention to p o vide'y a completely -iiexibles'ystem ofgselecting land mixingseismic signals. l v'lit' sa stillfurther object of the invention to provide" a "system -xvhicliis'frelialrile in operation, Vre"i :ii ii'iesw a 1 minimum' Y umher of" circuit compo- "nentsfand Which'i's of rugged cons'tr'uction.` 'Various other objects, advantages and'fefatures of the' invention Will' become" apparent from 'the .following detailedndesoriptinftaken in conjuncti'oiwith the abjconpa'nyijgdrawings, in which: V "Fligu'r'e is ablocl Adiv Vrain"oftheiiovelseisinic system, With a 'number of 'the transmitter-'seis- Vrnometerunits vomitted for purposes'fof'clarity;

` ligure is 'a" sch'en'a'tic i'iefv1 ofr an'array of seismoineter-trans'mittei' units; "FigureS is aA's'che'rnati'circuit diagram of a mixing circuit used inthe receiver of Figure 1 and. ,Figure 4 is a .schematic-@Weit diagram 0f a Inpsiiiefi mixingcircuit.

. Referring novvA 4 to the drawings .in detail, and particularly to, Figure l, thesystemincludes a .number of transmitterrseisniorneter units l0, each' ofwhicliincludes aSeisnio'lfreter I I` for Conrelies fseiemreves iedident-l upon the .seismometel intogeleetrial-voltages .representative thereof. The voltages produced by each of the seismometers II are fed to a transmitteri2 associated therewith which radiates a radio wave modulated by the seismometer output voltage. Although four such units are shown in the drawing, it will be understood that any desired number may be used depending upon the nature and extent of the region to be surveyed. The radio waves produced by the transmitters I2 are all of different frequencies which, however, are quite close together in the frequency spectrum, due to governmental and practical requirements; Preferably and advantageously, there are equal frequency differences between adjoining Waves in the frequency spectrum, and thevarious transmitter-seismometer units are disposed in a regular geometrical pattern. While many orderly geometric arrangements are possible; for example circular, rectangular and linear arrays, to name a few, a suitable arrangement for a system using sixteen seismometers is shown in Figure 2, in which it will be noted that there are four rows A, B, C, and D of seismometers with four seismometers I, 2, 3, and 4 in each row. As a specific example, the frequencies of the respective transmitters may be as indicated in the following table:

Table I Referring again to Table I and Figure 2, if it were desired to combine the output of all the seismometers in row A, for example, a single local oscillator I1 may be used which is tuned to a frequency of 147.00 megacycles. As a result. two

' heterodyne signals are produced for each incom- Fromrthe foregoing table, it will be noted that there is a frequency difference of .25 megacycle between adjoining transmitter frequencies andA .that all of the transmitters have different fre` quencies. Thus, for example, the frequency of vtransmitter A3 (153.25 megacycles) is .25 megacycle greater than the frequency of transmitter A2 (153.00 megacycles). Similarly, the frequency of transmitter D I (155.75V megacycles) is .25 megacycle greater than the frequency of transmitter C4 (155.50 megacycles). q Y

The signals from all transmitters are fed to a multi-channel receiver generally indicated by reference numeral I4, Figure 1. tion of this receiver includes a radio frequency amplier I5 which amplies all the incoming signals without separating them. From the amplifier I5, the signals are fed to a mixing circuit I6 wherein they Vare heterodyned with the output r" signals of a local oscillator unit, which may include one or more local oscillators I1, I8, I9, and 20. The resultant beat or heterodyne signals are fed from the mixing circuit to one or more intermediate frequency amplifiers 22, 23, 24, and 25, a preselected signal or group of signals being passed by the input circuits of each intermediate frequency amplifier. The signal from each intermediate frequency amplier is fed to a second detector-seismic amplifier unit 25 and the output of each of the units 26 is fed to a common recorder unit 21.

In accordance with the invention, by varying the output frequency and number of local oscillators, I provide an array of input signals to the intermediate frequency amplifiers, and each of the intermediate frequency ampliers is tuned so as to select a different predetermined combination of incoming signals which are amplified, rectified, and passed to the recorder.

The input porverodyne signal having a frequency further be apparent ing signal, one heterodyne signal having a frequency equal to the sum of the signal frequency and beat oscillator frequency, and the'v other hetequal to the difference between the local oscillator frequency and signal frequency. The difference frequencies are listed in the following table:

Table II DIFFERENCE FREQUENCIES PRODUCED BY LOCAL OSCILLATOR FREQUENCY OF 147.00 MEGACYCLES Assuming that intermediate frequency amplifier 22 is tuned to pass a frequency band of 5.75 to 6.50 megacycles, it will be apparent that the heterodyne signals representing the output of seismometers AI, A2, A3, and A4 will all enter the amplifier 22 while the remaining signals will be rejected. Accordingly, the signal passing into intermediate frequency amplifier 22 represents the combined output-of the seismometers in row A and this combined output-is recorded as a. single trace by the unit 21. Similarly, if intermediate frequency amplier 23 is tuned to a frequency of 6.75 to 7.50 megacycles, the output voltage thereof represents the combined output of the seismometers in row B and the combined voltage is recorded as another trace by unit 21. It will be apparent that various other combinations of seismometer outputs are ing the intermediate frequency amplifier units to preselected frequency ranges. For example, an intermediate frequency amplifier sharply tuned to a frequency of 8.50 megacycles will pick up only the output of seismometer C4. It will that similar results may be achieved by using a smaller or larger number of seismometers and that the frequencyof or frequency difference between the signals may be varied without changing the operation of the invention. However, aswill be seen hereafter, it is desirable that the frequency differences between the transmitters be kept uniform.

In accordance with the invention, the combinations of preselected groups of seismometer signals may also be accomplished by providing a plurality of local oscillators whose frequencies are so chosen that a plurality of intermediate frequency signals of substantially the same wave length are produced in the mixing circuit, these signals representing the output of preselected groups of seismometers. In particular, assuming that there is an equal frequency difference between adjoining transmitter signals, and that a plurality of local oscillators are provided whose signals have a frequency of the frequency difference between adjoining transmitter signals, at each of several wave lengths there will be a group of signals from transmitters which are separated by a frequency difference equal to that between the local oscillator signals.

The foregoing manner of operation of the syspossible by tuno difference which is an integral multiple in Figure 2..-. In thisexamnle. thefpur-lqeel 05.@.11- -latorsmf Eigure .l are. tuned. respeetivelyf.- .tOi-.frequenciesbf 147.00, 148.00, 149.00,- and 150.00 meg acycles-while the transmitters are 12H11ed.t.0f.h.e frequencies listed in..-Tab1e-.I- It..w i11 haunted -that-.the frequency difference between fthe-local nsilla.. .xt-...Signals tlilunegacyle) .iS-.219. ntegll multlpleof the-frequency difference between adjoining-radio wa-ves..1.25.` megacycle.. Table I). Llhe intermediate frequency diierene .signals produced the -interaction` of.-l the fur...10ca1 LOCAL osoILLA'ro E FREQENC Y, 148.00 M E clic rotas l 4.15 5.00 5.25 i 5.50 5.15 s. 0 0 5 25 5. 5o e. 1. 00 1. 25 1. 5o l1.15 s. 0o s 25 s. 50

Loo-ALvoseI-LLA-'ro R E nEQU'ENominoo ME GAC Y'oEs 2.15 3. 0 0 3. 2 5 3. 5o a. 15 4. 00 4. 25 4. 50 4. 15 s. 00 5. 25 Y5.50 5. 15 o. 00 5. 25 c. 5 0

r @.Fromfthe. aboye table, it w ill be noted that the .interaction of the 1 4 8 .00. megacycle. signal .with .the .signalfrom transrnitter.. 03, whose, Y freguent-.y 4.ls-.155.25 megacycles, produces arr intermediate frequency signal at,.. 7. 25 megacyclesf while, as anotherA example.. tha l50.09.- n1egacycl e signal interaots with the outputoftransmitter AI ,-Whose .ff-requeny.- ...i5- 2.75, .-megacycles.A to produce .an intermediate-frequency signal at 2. f75 megacycles.

llo

.-Y,..-It wil-1 loe apparent. that. an intermediateifre- Queries'.` ampliier .tuned sharply. to. a frequency .0f 5.7.5 megacycles willv Dik up the combined outputsof. transmitters AI, Bl, C I,l and DI, that is, .the transmittersof the .first column, Figure 2.

-sincegan intermediate frequency signal of-5.75

megacycles is; produced .bythe interaction oteaoh .-.0ff.zthese transmitter frequencies with one of the decal-.oscillator irequencies. Similar1y,ar i intermediate; frequencyamplier sharply tuned to .a frequency off- 6.50 megacycles combined output of-.all-the transxnitters` in the fourth column, Figure 2, that is, transmitters 'A5, rfBffyand D4.v Similarly, the outputffrom the .transmitters in .the secondand third -coluinnsmay A.becombinedby providing` intermediate frequency.

.amplilers sharply tuned to Afrequenciesfoi 6,00 .and 6.2.5 .meeeeycles respeetively.. Alternatively :the outputof. various .other greens. .0i tren-Selities :lzm'uahe-fpomblled, Eorexaljiplean inter ediate of 4.25 megacycles will pick up the combinedl out' wi ll pick up the frequency amplifier. ffrable IV, it will benoted that an. .intermediate .--irequency amplifier tuned to passfrequenoies of :6.00 .to 6.3.0 megaoycles, for example, will Lplck up put. of transniilztii's A3 As another ample, aninterniediatefrq to the range of 7.75 .tLOO me cles. will pick up the combined outplit of transmitters Cl, C2, DI, and D2.

It will be understood, however, that the frequency differences between the signals need not necessarily be equal nor is it necessary that there be equal frequency .differences between the 1ocal oscillator frequencies, in order to obtain certain of the results of the present invention. In seismic exploration, it is oftentimes very der sir-able that the signals be picked up in groups of two, particularly where. theoutput of a translational geophone is to becombined with the out.- put of a rotational geophone .to eliminate the eiects .of surface waves.. In .acordance .with .the invention, the transmitter and .local oscillator .'freguencies. may. be .readily chosen to pick upftloe signals in groups .of two. If there are equal fre?- -quency differences between the Vtransmitt'e'r. `signals.. this maybe done by. utilizing two.lo calosoilgt latorshavin a Iredu'ency .diier'ence equal .tothe frequency difference between 'adjoining transmitter. signals. In the specific ekample of FigurefZ,

.this result may be obtained by utilizing two local oscillators tuned to frequencies of 147,09 and 147.2 5 megacyoles. The resulting 1-interiediatla signal frequencies are shown by Table IV:

'5. 15 "5.00 fe. 2'5 o. 15 '1. Vo0 1.- 25 v. 50 .1.15. .8. 0o 8:25 8.50 1 5 4 n.00 0.25 5.50

BEAT osolLLATO R .l'liEQu'ENoix '141.25 iuEulicYoLns From Vthe foregoing. tab1e,it will be apparent .that the transmitter. signalsmay.be-readilyflm- Ybined in groups ofv two. For example, an vintermediate frequency.-A amplifier tuned. to .5.75 .inega- .cycles will pick up the combined outputs of. trans.- .mittersAl Y(Table IV. top portion). anda?, .('Ilable bottom portion). .An intermediate .frequency .amplifier sharply. tuned..to..8.50 megaoyclesl will pick up the combined outputs of transmitters C@ (Table IV, top portion) .and DJ. (.TableIV. bQt- .tom portion). .It will further be. apparentthat Vcombined signalsof anytwo adjoining transmit- 4vters maybe readily chosenby properly.. tuning one, of the intermediate ...frequency amplifiers. .Furthen several. groups. may .be .selected at one time. `and their respective .combined signals vmay be recorded by the unit. 21.. by providing a separate detector and amplifier unit for each intermediate Also,..in connection with the combined output of seismometers A2, A3 AgTa-ble `Iii/*, top portion), and A4, and BL (Table 1V,.bottom portion). Various othergroups may Eobviously oe chosen by suitable. selection of the intermediate 'frequency band pass.4 ranges..v

desired to pick up the combined signal of the transmitter-seismometer units along a diagonal 'of Figure 2, local oscillator frequencies may be selected as shown in Table V:

Table V i Local OS- igual Freintermediate seismometer queues, I Frequency 152. 75 147.00 5. 75 l 154. 148. 25 2 5. 75 i 155. 149. 50 i 5. 75 )56. 50 150. 75 l 5. 75

From this table it will be observed that each of the transmitters along the major diagonal produces an intermediate frequency signal at 5.75 lmegacycles by interaction with one of the local oscillator signals. These signals may then be picked up as a group by an intermediate frequency ainplier sharply tuned to a frequency of 5.75 megaeycles. Similarly, by making the proper Ichoice of transmitter frequencies, local oscillator frequencies, and intermediate frequency tuning, any desired combination of transmitter outputs may be provided for each of the intermediate frequency amplifiers 22. In addition, any desired number of intermediate frequency amplifiers may be used and the outputs of as many groups as desired be simultaneously recorded by the unit 2'?.

Referring again to Figure l., the circuit elements of the combination are all of conventional construction with the exception of mixing circuit I6 and the features claimed in my aforesaid copending applications, Serial Nos. 41,337, 30,257, and 27,590, now Patent Nos. 2,479,772, 2,494,830, and 2,557,310 respectively. However, insofar as I am aware, the disclosed combination of elements is new. Hence, the conventional elements themselves need not be described in detail. Two suitable embodiments of the mixing circuit i6 are shown in Figures 3 and 4.

Referring to Figure 3, a set of input terminals 30, 3i is provided for receiving the output of radio frequency amplifier l5, and four sets 32 and 33,

` 34 and 35, 36 and 31, 38 and 39, of input terminals are provided for receiving the outputs of the respective local oscillators l1, I8, I9, and 20. Terminals 3|, 33, 35, 31, and 39 are grounded while each of the other terminals is connected through a separate decoupling impedance to a common conductor. This conductor is connected to the 'control grid of a pentode 42, the anode of which .is connected through an intermediate'frequency amplifier coupling network 43 to a positive terminal 44 of a suitable power source, the negative terminal 45 of which is grounded. The intermediate frequency coupling network is preferably similar to that disclosed in my copending application Serial No. 41,337, now Patent No. 2,479,772 and effectively prevents interference between the different signals or groups of signals Picked up by the respective intermediate frequency amplifiers 22, 23, 24, and 25. The suppressor grid of tube 42 is connected by a lead 46 to the cathode thereof, and the cathode, in turn, is connected to ground through a resistor 41 which is by-passed by a condenser 48. The screen grid of tube 42 is connected through a resistor 49 to positive terminal 44 and the screen grid is also grounded by a filter condenser 50. Signals appearing across the respective sets of input terminals are mixed by the disclosed circuit and produce groups of intermediate frequency signals, as described in connection with the specific example of Figure 2. Desired individual signals or groups thereof are selected by the tuned intermediate frequency input circuits connected in the anode circuit of tube i12 through coupling network 43.

In Figure 4, I have shown a suitable mixing circuit for use with two local oscillators. The output of the first local oscillator is fed to terminals 52 and 53, the output of the second local oscillator is fed to terminals 54 and 55, while the output of the radio frequency amplifier I5 is fed to terminals 56 and 51. Terminals 53, 55 and 51 are grounded while the other terminals are connected throughthe respective coupling condensers 5t to the first, second and third grids of sistor 54 which is shunted by a filter condenser 65. The screen grid of tube 59 is connected through a resistor to terminal 6| and a lter condenser El is provided for grounding the screen grid. The operation of the circuit of Figure 4 is similar to that of Figure 3 except that the signals are mixed by control of the electron current flowing through tube 59 rather than by beingr interconnected at a common conductor, as in the circuit of Figure 3.

It will be apparent that I have described a system for providing signals of different frequencies corresponding to the outputs of a group of seismometers and thatI have disclosed means for selecting any desired groups of these signals Aand recording them separately to provide indications of the combined seismometer outputs. This provides a iiexible and highly versatile mixing system and many useful functions can now be obtained which have not heretofore been possible.

While the invention has been described in connection with a present, preferred embodiment "thereof, it is to be understood that this description is illustrative only and'is not intended to limit the invention, the scope of which is defined by the appended claims.

h Havingdescribed my invention, I claim:

u -quency signals and to reject all other intermediate frequency signals, a detector fed by said amplifier, and a seismic amplifier-recorder unit fed by said detector.

2. In a radio seismic system, a plurality 'of seismometers for converting seismic waves into electrical voltages representative thereof, a pluralityv of transmitters tuned to closely adjacent frequencies and connected to the respective seis- 'rnometers to radiate Vradio waves modulated by said electrical voltages, `and a receiver including a -local oscillator unit for producing at least two signals of different frequencies, the frequency difference between said two signals being equal to the frequency difference between a preselected pair of the radio waves radiated by said transmitters, a mixing circuit for heterodyning said radio Waves with said pair of local oscillator signals, thereby to produce a plurality of intermediate frequency signals having a common frequency, an intermediate frequency amplifier fed by said mixing circuit and tuned to said common frequency, a detector fed by said amplier, and a seismic amplifier-recorder unit fed by said detector.

3. In a radio seismic system, a plurality of radio transmitter-seismometer units for radiating radio waves modulated by voltages representative of seismic waves, said radio waves having different but closely adjacent frequencies, a receiver including a plurality of local oscillators whose signals have a preselected frequency relationship to predetermined ones of said radio waves, a mixing circuit for heterodyning said radio waveswith the output of all of said local oscillators to produce intermediate frequency signals, whereby certain of the intermediate frequency signals produced by the interaction of said predetermined radio waves with the outputs of the local oscillators have a common frequency, an intermediate frequency amplifier fed by said mixing circuit and tuned to said common frequency, a detector fed by said intermediate frequency amplifier, and a seismic amplifier-recorder unit fed by said detector.

4. In a radio seismic system, a plurality of radio transmitter-seismometer units for radiating radio waves modulated by voltages representative of seismic waves, said radio waves having different but closely adjacent frequencies with an equal frequency difference between adjoining radio waves in the frequency spectrum, a receiver including a plurality of local oscillators whose signals have a frequency difference which is an integral multiple of the frequency difference between adjoining radio waves, a mixing circuit for heterodyning said radio waves with the output of all of said local oscillators to produce intermediate frequency signals, thereby to produce several groups of intermediate frequency signals the signals in each group having a common frequency, an intermediate frequency amplifier tuned to said common frequency, a detector fed by said intermediate frequency amplier, and a seismic amplifier-recorder unit fed by said detector.

5. In a radio seismic system, a plurality oi radio transmitter-seismometer units for radiating radio waves modulated by voltages representative of seismic waves, said radio waves having different but closely adjacent frequencies, with equal frequency differences between adjoining Waves in the frequency spectrum, and a receiver including a pair of local oscillators, the frequency difference between the local oscillator signals being equal to the frequency difference between adjoining waves, a mixing circuit for heterodyning said radio waves with the output of all of said local oscillators to produce intermediate frequency signals, whereby a pair of intermediate frequency signals of a common and characteristic frequency is produced for each pair of adjoining waves, an intermediate frequency amplifier fed by said mixing circuit and tunable to said characteristic frequency, a detector fed by said intermediate frequency amplier, and a seismic amplifier-recorder unit fed by sa-id detector.

6. A receiver for a radio seismic system comprising, in combination, a radio frequency amplifier adapted to receive and amplify a plurality of modulated carrier waves of different frequencies, a plurality of local oscillators for producing signals of dierent frequencies, a mixing circuit for combining the output of all of said oscillators with the output of said radio frequency amplifier, an intermediate frequency amplifier fed by said mixing circuit, a detector fed by said intermediate frequency amplifier, and a seismic amplifier-recorder unit fed by said detector.

7. A receiver for a radio seismic system comprising, in combination, a radio frequency amplifier adapted to receive and amplify a plurality of modulated carrier waves of different frequencies, a plurality of local oscillators for producing signals of different frequencies, a mixing circuit for combining the output of all of said oscillators with the output of said radio frequency amplifier, a plurality of intermediate frequency amplifiers tuned to different frequencies, a coupling network for feeding a portion of the output of said mixing circuit to each of said intermediate frequency amplifiers, a detector fed by each of said intermediate frequency amplifiers, a seismic amplifier fed by each detector, and a common recorder unit for all of said seismic amplifiers.

8. A receiver for a radio seismic system comprising, in combination, a radio frequency arnplifier adapted to receive and amplify signals of different frequencies, a plurality of local oscillators for generating signals of different frequencies, a mixing circuit for combining the output of all of said local oscillators with the output of said amplifier, and a plurality of tuned intermediate frequency amplifiers fed by said mixing circuit, the frequency of said local oscillators and said tuned amplifiers being variable to permit selection of a predetermined group of signals for each tuned amplifier, a detector fed by each intermediate frequency amplifier, a seismic amplifier fed by each detector, and a common recorder unit for all of said seismic amplifiers.

GLEN PETERSON.

References Cited in the flle of this patent UNITED STATES PATENTS Minton June 3, 

